The present invention relates to telephone circuits for generating signals designed to display the relative charges of a telephone call in progress to the subscriber and in particular to telephone circuits, which may be monolithically integrated, for generating control signals for displaying the charges to the subscriber and which is designed to form, together with the speech circuit of the subscriber's line, an interface between the subscriber's telephone line and exchange components.
A subscriber's telephone line is supplied by a D.C. voltage generator, in series with which there are connected other voltage signal generators which generate speech band conversation signals, ringing signals and the signals designed to display the charge to the subscriber.
Both the speech circuit and the ringing circuit of the subscriber's telephone set are connected to the terminals of the line, as well as to other possible devices, which may be connected internally or externally to the set itself. For example, a display may be connected to the telephone set for displaying the accrued total of the charges of the telephone call in progress which is carried out by the appropriate exchange components.
The subscriber's charge display is, as mentioned above, driven by the telephone exchange, via the same telephone line, by means of control signals, normally called "charging signals".
These control signals must have a high amplitude with respect to the maximum amplitude of the speech signals, but cannot be embodied as signals having a pulse waveshape since the leading and trailing edges of such pulses would lead to harmonics which are likely to disturb not only the user of the line, but also the users of neighboring lines. In order to avoid disturbances on other neighboring lines, it is also necessary for the level of these "charging signals" in the line to have a low sensitivity to impedance variations in the line.
In practice, use is made of AC signals having a duration which is limited over time (approximately 100 msec) and a frequency (12 or 16 KHz) which is considerably higher than the frequency of the telephone speech signals (0.3-3.4 KHz), with relatively long leading and trailing times.
At present the "charging signals" are generally formed and supplied directly to the line by means of exchange devices of the electromechanical type comprising a resonant circuit coupled inductively, via a transformer, to the subscriber's telephone line and actuated at predetermined time intervals.
The leading and trailing edges of such charging signals normally have an exponential waveshape.
The solution described above is therefore comparatively costly and makes it necessary to tune the resonant circuit accurately to the required frequency.